This invention relates to an air separation plant.
Air separation plants in which the air is separated by rectification (i.e. fractional distillation) at cryogenic temperatures or by pressure swing adsorption are well known. A product of air separation, for example, oxygen or nitrogen, is often required at elevated pressure. On some occasions, the elevated pressure is greater than that at which the separation is performed. Typically, therefore, the air separation plant may include a product compressor.
U.S. Pat. No. 4,382,366 relates to an oxygen generator in which an oxygen product compressor is directly driven by a steam turbine. A waste nitrogen stream containing sufficient oxygen to support combustion is taken from the rectification column in which the oxygen product is separated and is without further compression introduced into a chamber in which combustion of a fuel gas takes place. The resultant combustion products are expanded in a turbo-expander. The stream supplied to the steam turbine is raised by heat exchange with the combustion gases exhausted from the turbo-expander. The oxygen product compressor, the air compressor of the oxygen generator, the steam turbine and the turbo-compressor are all coupled together. Such a plant cannot produce a nitrogen product in large quantities.
Air separation plant in which a single nitrogen product is produced at a rate of over 1000 tones per day at elevated pressure is well known. A large product nitrogen compressor is therefore required. Such a nitrogen compressor is conventionally driven by an electrical motor.
The need sometimes arises to vary the pressure at which the nitrogen product is produced. An example of such a need is in the nitrogen-enhanced recovery of oil or gas from, respectively, an oil field or a gas field. A large product nitrogen compressor driven by an electrical motor is relatively inflexible and is not readily suited to supplying the product at the different pressures that are typically needed over a prolonged period of time for the recovery of oil or gas.
An aim of the present invention is to provide an air separation plant which has an alternative means of driving a product nitrogen compressor better able to cope with a varying pressure demand.